From the point of view of his genes, there is no worse fate for a male animal than to invest time & energy in the raising of offspring that aren't his, i.e. when they were sired by another (and not closely related) male.
Quite literally, it is a fate worse than death (again, from the genes PoV, which of course genes do not have, but it can be useful to imagine they do). It's wasted effort - his time is better spent siring his own progeny and investing his energies in their survival.
Note: clearly, human Dads are able to transcend the selfishness that the genes would impose on them.
On the other hand, there is no sweeter result for a male's genes than getting a free ride, i.e. the host male impregnating some female such that the resultant offspring will be reared by some other (almost certainly unsuspecting) male. The lothario enjoys the milk (genetic legacy) without the cost (time & effort spent rearing the small genetic packages in which that legacy is manfested) of purchasing the bovine, and is free to repeat the strategy with other females and other 'duped' husbands - potentially vastly expanding his genetic output.
(Lest I be accused of casting the female as only a passive player; the ladies play
the game of maximizing genetic output for minimal effort just as well - just differently).
'Fathers' can follow a number of strategies to ensure that any offspring of their mate are indeed their own - including restricting access to their mates in order to prevent impregnation, combating any sperm able to get past such access controls, and killing offspring resulting from any sperm able to defeat the other controls.
Forced exclusivity in partner selection taken to the extreme.
Hmmm. Might an IDP want to impose an exclusive whitelist on its partner SPs? Either explicitly, or implicitly, through incentives or UI?
From the other side, under what conditions might an SP be willing to accept such a constraint (i.e. how good a deal would it have to be?)
Note: answering 'enterprise' doesn't count, as both IDP and SP are in the same policy domain.